This invention relates generally to digital circuits, and more particularly to a new and improved translation circuit for converting transistor-transistor-logic (TTL) signals to voltage levels that are compatible with emitter-coupled-logic (ECL) circuitry.
The explosive growth experienced in various of the electronic industries today has resulted in part from the ability to integrate more and more electronic circuitry on smaller and smaller areas of silicon. This integration stems not only from the unique techniques developed in recent years to fabricate electronic integrated circuits, but also from more efficient circuit design which also reduces component count. In addition to reducing the area needed to form a circuit on a chip, reducing component count through efficient circuit design often also results in improved operating speeds and lower power consumption.
Further, while recently developed semiconductor fabrication techniques and processes have achieved astounding success in raising the number of circuit components that can be formed on a given area of silicon, there are limits. Accordingly, when present fabrication techniques reach a limit as to the component count that can feasibly be formed on an integrated circuit chip of a certain size, efficient design techniques become important in the ever-present effort to put more and more circuits on the chip.
Turning to another aspect of digital systems that utilize bipolar technology, it is sometimes necessary to combine and use transistor-transistor-logic (TTL) with emitter-coupled-logic (ECL) on a single system. Alternatively, one digital system may be implemented in TTL, while another in ECL, and a need arises to connect the two together. Typically, TTL circuitry operates with higher voltage swings than that used in ECL in order to saturate or turn off various transistor elements. ECL uses transistors in the non-saturating mode to attain the high operating speeds for which it is known. That is, ECL configured transistors do not switch full-on or full-off, but swing above and below one or more given reference voltages.
In any event, whether TTL and ECL circuits are mixed or a TTL system must communicate with an ECL system, digital signals produced by TTL circuitry must be converted to ECL levels before they can be used by ECL circuitry. Often, this is accomplished by using a separate level shifting circuit at each input of the ECL circuit that receives a TTL signal.